At least two fluids may be mixed together to form a dispersion where one phase, a discontinuous phase, is dispersed in another, continuous phase, of a different composition or phase. An emulsion is a dispersion having two immiscible liquids where one phase is dispersed as droplets (the non-continuous or internal phase) in the other (the continuous or external phase). Thus, all emulsions are dispersions, but not all dispersions are emulsions. While emulsions are typically unstable thermodynamically, some form of mechanical, electrical and/or chemical treatment is often required for rapid and efficient separation of the immiscible phases.
In the petroleum industry, various operations including, but not limited to, exploration, production, refining and chemical processing of hydrocarbons including, but not limited to, crude oil, gas and their derivative products, routinely produce mixtures and dispersions of oil and water. Such mixtures may also contain other compounds, such as waxes, asphaltenes, various salts, suspended materials, biological surface active material from the ground, added surface active corrosion/scale inhibiting reagents, etc., which may vary from location to location. In addition, synthetic and natural surfactants, produced either in-situ or added in enhanced oil recovery techniques such as alkali-surfactant (AS) and alkali-surfactant-polymer (ASP) floods, can cause phase separation issues. Along with the presence of these other compounds, high shear and mixing forces may cause these oil and water mixtures to form relatively stable emulsions. Some such emulsions are water in oil emulsions, commonly referred to in the petroleum industry as “regular emulsions” where oil is the continuous phase. Others are oil in water emulsions, commonly referred to in the petroleum industry as “reverse emulsions” where water is the continuous phase.
“Breaking” an emulsion means to separate the oil-based liquid phase and the water-based phase. The term “breaking” implies that the emulsifying films around the droplets of water or oil are “broken” so that coalescing may occur and result in separation of the phases over time, for example, by gravitational settling or the application of an electrical field. For example, demulsifiers or emulsion breakers may be added to a fluid stream to break or separate an emulsion, whether normal or reverse, into its constituent phases. Clarifiers may be added to a fluid stream to break emulsions and separate the oil phase from the water phase, thereby, making the water “clearer.” There is known to be some overlap in the types of compounds which effectively demulsify and those which clarify, i.e., some compounds are useful for both demulsifying and clarifying oil-water emulsions and dispersions.
During the production phase of an oil well, formation water is also produced from the well in combination with the oil. Further, in many secondary and tertiary oil recovery techniques such as steam or water flooding, oil extraction from tar/oil sands and steam assisted gravity drainage (SAGD), large amounts of water are used to recover oil. In such circumstances, the oil typically comes out of the ground as an emulsion. To break this emulsion or separate the water-based phase from the oil-based phase, demulsifiers such as polyalkylene glycols (PAGs), block copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), and alkylphenol resin alkoxylates may be generally used. In some cases, water clarifiers are also added to the mixed production stream.
Once the emulsion is separated into an oil fraction and a water fraction, the produced water fraction (i.e., “produced water”) may contain oil entrained in the water-based phase. In other words, a “reverse” i.e., oil-in-water, emulsion is formed. This emulsion may appear as a turbid, sometimes brown or black phase, depending on the amount of impurities present. Such produced water may be generated in very large quantities (i.e., up to millions of gallons per day), and is usually either re-injected into the formation, or disposed of into the ocean. Governmental regulations, such as the US Clean Water Act and the US EPA Code of Federal Regulations in the United States, require reduction of the amount of oil content down to very low levels before the produced water may be discharged. Although the amount of oil permitted in discharged water under such regulations varies from jurisdiction to jurisdiction, the standard is generally very low, such as less than 29 ppm oil in water as an example. Further, with the increasing cost and regulation on fresh water use, low levels of oil are demanded for water reuse. The practice of reducing the oil in water is commonly known as “clarification,” and is simply the breaking of the reverse emulsion. Clarification of such demulsified oilfield water may involve the use of acrylate polymers, cationic polymers, cationic polyelectrolytes, and water-soluble amphiphilic polymers to flocculate suspended oily and particulate materials and, thereby, obtain clear(er) water.
Successful selection and use of effective polymer compounds to demulsify and clarify oil-water emulsions formed during petroleum industry operations may be very complex because whether or not a particular polymer will work depends on the geometry of the production system, the source of water, the nature of suspended solids, the composition of the oil, the nature of other reagents used, etc. Thus, there is no one single, universal solution for the practice of demulsification and clarification of oilfield emulsions. Depending on individual oilfields and the conditions involved, different chemicals, e.g. polymers, will provide optimum performance in different locations.
U.S. Pat. Nos. 4,855,060 and 5,006,274 to Baker Hughes Incorporated involve methods and additives for clarifying an aqueous system which may contain an oil-in-water emulsion, a dispersion of a non-aqueous discontinuous phase, without production of a resultant uncontrollable floc in the system, by contacting the system with an effective water clarifying amount of a composition having present a dithiocarbamate of bishexamethylenetriamine (BHMT). The composition is maintained in the system for sufficient time to effectively clarify the system without production of an uncontrollable floc.
One chemical solution to the problem of emulsification is disclosed in U.S. Pat. No. 5,100,582 to Bhattacharyya. This patent discloses polymeric demulsifiers for breaking water-in-oil emulsions, where the polymeric demulsifiers have random structures prepared using varying percentages of acrylic acid, methacrylic acid, ethyl hexyl acrylate, monomethyl acrylate and butyl acrylate as their effective constituents.
It would be desirable if compositions and methods were devised for more economical and/or more efficient treatment of combinations of oil and water for removal of the oil-based phase.